Normal is around 40 - 50 psi. Expect less with 300,000 miles. Worn crankshaft and connecting rod bearings are much more common causes of low oil pressure than pumps, but you may get lucky.
As for the gauge, unplug the sending unit, then check the gauge. If it still reads max, a wire is grounded; possibly pinched between something you reassembled. We all do that sooner or later. If the gauge goes to 0, try a new sending unit. Remove the old one first, then crank the engine about two seconds without starting the engine. That will wash out any debris that might be lodged in the passage, but it will make a small mess on the floor too.